The histamine receptors, H1, H2, H3 and H4 have been characterized by their pharmacological behavior. The H1 receptors are those that mediate the response antagonized by conventional antihistamines. H1 receptors are present, for example, in the ileum, the skin, and the bronchial smooth muscle of humans and other mammals. The most prominent H2 receptor-mediated responses are the secretion of gastric acid in mammals and the chronotropic effect in isolated mammalian atria. H4 receptors are expressed primarily on eosinophils and mast cells and have been shown to be involved in the chemotaxis of both cell types.
In the periphery, H3 receptor sites are found on sympathetic nerves, where they modulate sympathetic neurotransmission and attenuate a variety of end organ responses under control of the sympathetic nervous system. Specifically, H3 receptor activation by histamine attenuates norepinephrine outflow to resistance and capacitance vessels, causing vasodilation. In addition, in rodents, peripheral H3 receptors are expressed in brown adipose tissue, suggesting that they may be involved in thermogenesis regulation.
H3 receptors are also present in the CNS. H3 receptor expression is observed in cerebral cortex, hippocampal formation, hypothalamus and other parts of the human and animal brain. H3 receptors are expressed on histaminergic neurons and, as heteroreceptors, on neurons involved in other neurotransmitter systems, where H3 receptor activation results in presynaptic inhibition of neurotransmitter release. In the particular case of histaminergic neurons, H3 receptors have been implicated in the regulation of histamine hypothalamic tone, which in turn has been associated with the modulation of sleeping, feeding and cognitive processes in the human brain (see, for example, Leurs et al., Nature Reviews, Drug Discovery, 4, (2005), 107).
It is also known and has been described in the literature that histamine is involved in regulation of cognitive and memory processes in the human brain (see, for example, Life Sciences, 72, (2002), 409-414). Consequently, indirect modulation of histaminergic brain function through the central H3 receptors may be a means to modulate these processes. Different classes of H3 receptor ligands have been described and their use for neurological and psychiatric diseases has been suggested (see, e.g., US Patent Publication No. 20040224953, International Publication No. WO2004089373 and International Publication No. WO2004101546). H3 receptor antagonists may be useful for treating various neuropsychiatric conditions, where cognitive deficits are an integral part of the disease, specifically ADHD, schizophrenia and Alzheimer's disease (see, for example, Hancock, A.; Fox, G. in Drug Therapy (ed. Buccafusco, J. J.). (Birkhauser, Basel, 2003).
Imidazole H3 receptor antagonists are well known in the art. More recently, non-imidazole H3 receptor antagonists have been disclosed in U.S. Pat. Nos. 6,720,328 and 6,849,621, and in US Published Applications 2004/0097483, 2004/0048843 and 2004/0019099.
U.S. Pat. No. 5,869,479 discloses compositions for the treatment of the symptoms of allergic rhinitis using a combination of at least one histamine H1 receptor antagonist and at least one histamine H3 receptor antagonist.